1. Field of the Invention
This invention relates to a head driving apparatus for a thermal printer, and more particularly to a driving apparatus for moving a thermal head either longitudinally of a platen or perpendicularly toward and away from the platen in a thermal printer.
2. Description of the Related Art
Heretofore, thermal printers in which characters are printed on a thermosensitive paper by forcing the thermosensitive paper against a platen by a thermal head are popular as a recording means for process control and a character information output means for a microcomputer. Since a thermosensitive paper has the color forming feature, the conventional thermal printers can be small-sized and hence inexpensive.
For the printing action of the thermal printer, the thermal head is pressed against a thermosensitive paper on the platen in response to a command from a control unit, whereupon a carriage supporting the thermal head is moved horizontally on a guide rail parallel to the platen.
Upon termination of printing, the thermal head is brought away from the thermosensitive paper by a drive mean, and with the thermal head in this posture, the carriage is returned to its original or starting position at high speed. This cycle of procedures is repeated.
In this type of thermal printer, the thermal head is supported directly by a slider disposed in the carriage. The slider is slidably mounted on the carriage for movement perpendicularly toward and away from the platen.
The carriage is slidably held on a support shaft parallel to the guide rail is movable on the support shaft longitudinally of the platen. The movement of the thermal head toward and away from the thermosensitive paper on the platen, at the end of either forward or backward stroke of the carriage during printing, is accomplished by turning the support shaft in forward and reverse directions through a predetermined angle about its own axis.
In such a head driving apparatus of the conventional thermal printer, the head is in contact with the thermosensitive paper on the platen during the forward movement of the carriage. The head is moved away from the platen at the end of the forward movement, i.e., upon completion of each printing action on the corresponding line, whereupon the carriage is returned to its original position at a speed higher than that during the forward movement and then restarts its forward movement from the original position to print the next line. This procedure is repeated as many times as there are the succeeding lines.
With this driving arrangement, since the speed of returning the carriage during the backward movement, in which no substantial printing action is performed, is increased, it is possible to improve the efficiency of printing by some degree.
However, this conventional printing method requires two kinds of separately controlled drives for the longitudinal movement of the thermal head forwardly and backwardly along the platen and the perpendicular movement of the head toward and away from the platen. Consequently, two separate drive motors must be used for driving the head in the two different directions perpendicular to one another; this makes the head driving mechanism rather complex and large-sized so that a raise of cost of production cannot be avoided.
At the start of the forward stroke of the carriage, the support shaft of the carriage is turned through a predetermined angle toward the platen to allow the slider to move toward the platen under the bias of the spring, thereby bringing the thermal head into contact with the thermosensitive paper. At the termination of the line printing, the support shaft is turned reversely to move the slider away from the platen against the bias of the spring, thereby bringing the head out of contact with the thermosensitive paper.
Accordingly, during the forward and backward movement of the carriage after the angular movement of the head toward and away from the platen, the spring-biased slider and a pawl mechanism are balanced.
As a result, during the forward and backward movement of the carriage, the biasing force of the spring urging the slider is unnecessary exerted on the support shaft via the pawl mechanism, which would occasionally be a cause for a raise of consumed electricity, a reduced life of the parts, noise, a fault or other trouble.